Many types of pneumatic hammer are available and are used, particularly but not exclusively, in the drilling of blast holes in mining applications. The applicants' earlier South African Patent No. 81/3065 describes such hammers and the contents of the specification of this earlier patent is incorporated herein in its entirety by reference.
Generally, a pneumatic hammer of the type described, also known as a down-hole hammer drill, comprises a hollow casing with operatively upper and lower ends. A piston is slidingly received within the casing to reciprocate between upper and lower pressure chambers defined within the casing. The hammer also has a bit assembly at the lower end of the casing and a backhead assembly at the upper end of the casing. The bit assembly is axially displaceable with respect to the casing. The piston of the hammer has a striking face at an end thereof proximate the bit assembly. The striking face of the piston is configured to strike a contact surface of the bit assembly, thereby urging the bit assembly to be displaced axially with respect to the casing and into the medium being drilled.
Generally, the contact surface of the bit assembly is narrower in diameter than the interior diameter of the hammer casing and the striking face of the piston has a diameter substantially similar to that of the contact surface of the bit assembly. It will be appreciated by those skilled in the art that in this configuration undesirable lateral forces may be generated on impact between the piston and the bit assembly causing them to be displaced laterally within the casing, thereby causing wear on the casing and sliding components at the points of contact thereof.